[Memnoch]

And in case you need more of a challenge after dustin' off the old tarrasque, say hello to Tiamat, ruler of the First Plane of the Nine Hells:

3rd Ed Stats - quite lengthy

Tiamat
Colossal Dragon
Hit Dice: 49d12+588 (906 hp)
Initiative: +4 (Improved Initiative)
Speed: 40 ft., Fly 150 ft. (clumsy), Sw 40 ft.
AC: 50 (-8 size, +48 natural)
Attacks: 5 Bites, +60 melee; 2 wings, +55 melee; sting, +55 melee
Damage: Bite, 4d6+19; wings, 2d8+9; sting, 4d6+9 plus poison
Face/Reach: 15'x40' / 15'
Special Attacks: Breath weapons, frightful presence, sound imitation, spells, spell-like abilities
Special Defenses: Scent, spell resistance 30, damage reduction (25/+4), immunities, see invisibility, keen senses, water breathing
Saves: Fort +39, Ref +27, Will +34
Abilities: Str 49, Dex 10, Con 35, Int 28, Wis 25, Cha 28
Skills: Alchemy +35, Bluff +61, Concentration +63, Diplomacy +61, Gather Information +59, Intimidate +61, Knowledge (Arcana) +31, Knowledge (Dragonkind) +34, Knowledge (History) +31, Knowledge (the Planes) +31, Knowledge (Religion) +31, Listen +61, Scry +61, Search +61, Sense Motive +59, Spellcraft +61, Spot +61, Wilderness Lore +33.
Feats: Alertness, Blind-Fight, Combat Casting, Hover, Improved Critical (Bite), Improved Initiative, Expertise, Fly-By Attack, Power Attack, Quicken Spell-like Ability (domination), Snatch, Wingover
Climate/Terrain: Any land and underground
Organization: Solitary (1), Troupe (Tiamat and 1d6 chromatic dragons of age categories d8+4)
Challenge Level: 25 (solitary)
Treasure: When traveling, Tiamat carries her magic items and 2d12+2 gems of various sizes. Her lair contains 2d4 level 20 treasures.
Alignment: Always lawful evil
Advancement Range: N/A

Combat:

Multiple Heads: As shown above, Tiamat gains five bite attacks per round. A head can be severed with a slashing weapon if the weilder inflicts a single blow causing 185 hp damage.

Breath Weapons (Su): Must wait d4 rounds between using breath weapons. As she has five heads, each head can deliver a different breath weapon. DC 46 Reflex save for half damage for each type of breath weapon:

Cold: 12d6 damage
Acid: 24d4 damage
Gas: 24d6 damage
Lightning: 24d8 damage
Fire: 24d10 damage
Frightful Presence (Ex): Can cause "panicked" or "shaken" state in creatures under 49 HD within a 480' range (DC 43 Will save negates for one day).

Sound Imitation (Ex): Tiamat can mimic any sound or voice. DC 43 Will save to detect.

Spells: Tiamat is a 20th-level sorcerer and a 20th-level Cleric with access to the domains of Evil and Law.

Spell-like Abilities: In addition to her spells, Tiamat can cast each of the following three times per day as a 20th-level caster: command plants, control weather, darkness, domination, fog cloud, gust of wind, mirage arcana, plant growth, suggestion, summon swarm, veil, ventriloquism. DCs against these spells are 19 plus the spell level. She can corrupt water once per day. She can charm reptiles three times per day (as mass charm spell, DC 27 Will save negates).

Poison (Ex): Stung creatures suffer 3d6 temporary Constitution damage (DC 46 Fort save to negate). After 1 minute, make a second save or suffer an additional 3d6 temporary Constitution damage.

Water Breathing (Ex): At will, always active.

Magic Items: Tiamat typically carries these items (not included in stats): amulet of the planes, +8 bracers of armor, carpet of flying, cloak of displacement, crystal ball with detect thoughts, darkskull, iron bands of Bilarro, iron flask, orb of storms, portable hole, +5 ring of deflection, +5 ring of resistance, rod of rulership, rod of splendor.